The invention generally relates to combining frequency multiplication and modulation (linear modulation, for example).
The generation of high frequency carrier signals (microwave carrier signals, for example) for use in communication systems (wireless communication systems, for example) presents challenges. For example, the implementations of microwave voltage controlled oscillators (VCOs) in integrated circuits makes its increasingly difficult to make these VCOs stable and able to produce clean carrier signals, especially in view of the trend to make these VCOs operate at ever-increasing frequencies. Furthermore, it is more difficult to design the phase locked loops of these VCOs for such high frequencies.
For these reasons, a modulation system 10 that is depicted in FIG. 1 may be used. In the system 10, a local oscillator 12 (a VCO, for example) generates a signal (a sinusoidal signal, for example) that has a fundamental frequency that is a fraction (one third, for example) of the frequency of the carrier signal. This signal from the oscillator 12 is received by a frequency multiplier 14 that multiplies (multiplies by three, for example) the frequency of this signal to produce the carrier signal, a signal that has a fundamental frequency that is three times the fundamental frequency of the signal from the oscillator 12. This carrier signal may be used by a modulator 16 to modulate an input signal at the carrier frequency to produce a modulated signal. Thus, the design of the oscillator 12 is simplified due to its lower frequency of operation, as compared to an oscillator that directly generates the carrier signal.
However, a difficulty with the modulation system 10 is that the frequency multiplier 14 may introduce a significant amount of spectral energy into the carrier signal at frequencies other than the carrier frequency. This distortion of the carrier signal may, in turn, expand or distort the modulated signal. Although it may be possible to pre-distort the original modulation to compensate for the distortion that is introduced by the frequency multiplier 14, this pre-distortion may not always be desirable for various reasons.
Thus, there is a continuing need for a technique and/or arrangement to address one or more of the problems that are stated above.